1. Field of the Invention
The present invention relates to liquid-crystal display (LCD) devices. More particularly, the invention relates to a reflective type LCD device using a front light and a semi-transmissive type LCD device using a back light, and a method of fabricating the devices, which reduces moire (interference) to improve the display quality.
2. Description of the Related Art
As known well, LCD devices are classified into the “transmissive type” and the “reflective type”. With the transmissive type LCD device, “an illumination source (a back light)” is required. Planar light is illuminated to the LCD panel from its backside with the back light and is temporally and spatially modulated by the LCD panel, thereby displaying information on its screen. On the other hand, with the reflective type LCD device, any dedicated back light is not incorporated, where ambient light is utilized to display information on its screen. Thus, the reflective type LCD device is unable to be used in the dusk or ill-lighted places. Generally, ill-lighted places are more popular than well-lighted ones and therefore, it is said that the display performance of the reflective type LCD device is less than the transmissive type LCD device.
The feature or advantage of the reflective type LCD device is that the back light which generates the majority part of electric power dissipation of the LCD module is unnecessary, or continuous illumination is unnecessary. Taking the advantage of low power dissipation into consideration, the device of this type is suitable for battery-driven portable electronic apparatuses, such as portable telephones and Personal Digital Assistants (PDAs). In addition, the device of this type has an advantage that the body is compact because the LCD module is thin.
Although the reflective type LCD device displays information on its screen using ambient light, it is popular that a light source called the “front light” is incorporated into the said device. This is because this device may be used in places where no external light is expected. The “front light” has a structure having a front optical guide plate and a small auxiliary light source mounted on the side of the plate.
Another type of the reflective type LCD device is the semi-transmissive (i.e., transmissive and reflective) type. With the semi-transmissive type LCD device, minute openings are formed to penetrate the reflector plate in such a way as to form a grid of dots, thereby making the said plate semi-transmissive. Therefore, the reflector plate serves as an optical diffuser plate for diffusing the light from the back light in ill-illuminated places or as an optical diffuser and reflector plate in well-illuminated places.
The reflective type LCD device comprises a reflector plate having diffusiveness, which is located behind the liquid crystal layer. The reflector plate may be placed inside or outside the liquid crystal cell. With the reflective type “color” LCD device, the reflector plate is placed outside the liquid crystal cell, where the microscopic shape and/or distribution of the bumps and holes in the surface of the reflector plate is/are controlled to optimize the distribution of reflected light. Thus, the degree of brightness is increased in a particular direction.
A metal electrode formed on the rear-side glass plate may be used as the reflector plate. This is to raise the reflectivity of the reflector plate in the reflective type LCD device by reflecting the light incident on the reflector plate as efficiently as possible. For example, the Japanese Non-Examined Patent Publication No. 8-101384 published on Apr. 16, 1996 discloses a method of forming a reflector electrode having the function of a reflector plate and the function of an electrode with aluminum (Al) having a high reflectivity and a low resistivity. Moreover, a method of displaying information by using the liquid crystal cell (in which bumps and holes are formed in the surface of the reflector electrode to generate a light scattering function), the phase plate, and the polarizer plate has been known. A method of forming the bumps and holes in the surface of the reflector electrode by a melt process has been known as well (Refer to the Japanese Non-Examined Patent Publication No. 2000-330104 published on Nov. 30, 2000).
FIGS. 1 and 2 schematically show the structure of a prior-art reflective type LCD device. FIG. 1 is a conceptual cross-sectional view showing the whole structure of the device and FIG. 2 is a conceptual plan view of the device seen from the viewer's side.
As shown in FIG. 1, the prior-art device comprises a reflective type LCD panel 11, a polarizer section 12, a light source 21 for the front light, and an optical guide plate 22 for the front light. Here, the panel 11 has a reflector plate with a surface irregularity (not shown) incorporated therein. The polarizer section 12 has a structure that a half-wave (λ/2) plate and a quarter-waver (λ/4) plate are stacked on a polarizer film.
The front light, which includes the light source 21 and the optical guide plate 22, is located in the front side (i.e., the viewer's side) with respect to the panel 11. While the front light is turned on, the guide plate 22 receives the light emitted from the source 21 such as a Light-Emitting Diode (LED), a Cold Cathode Fluorescent Lamp (CCFL), or the like. The plate 22 uniformizes the light in its inside to generate planar light and then, irradiates the light thus generated to the panel 11. Moreover, the panel 22 transmits the light reflected in the plate 22 to the viewer's side. If the LCD device is designed for PDAs, it is popular that a touch panel (not shown) is additionally mounted over the front light on the viewer's side.
Next, the propagation status of light in the guide plate 22 of the front light is explained with reference to FIG. 3.
As seen from FIG. 3, on the viewer's-side surface (i.e., the emission surface) of the guide plate 22, propagation parts 22a for allowing the light from the source 21 to propagate toward the viewer's side and reflection parts 22b for reflecting the light from the source 21 toward the panel 11 are formed. The combination of one of the propagation parts 22a and an adjoining one of the reflection parts 22b thereto constitutes an element termed a “prism”. Therefore, on the viewer's-side surface of the plate 22, a plurality of prisms is periodically arranged to form a “prism array”. Moreover, the groove formed in each of the prisms is termed a “prism groove”. The period of the prism array is equal to the pitch P of the prism grooves.
When the guide plate 22 receives the light emitted from the source 21 by way of its incidence surface 22c, the plate 22 uniformizes the light thus received in its inside with the reflection parts 22b, generating planar light. Then, the plate 22 emits the planar light to the panel 11 located to the opposite side of the viewer's side. On the other hand, the plate 22 transmits the light reflected by the reflector plate 113 located in the LCD panel 11 toward the viewer's side by way of the propagation parts 22a. 
Usually, the emission surface 22d of the guide plate 22, which is formed on the side of the panel 11, is covered with an anti-reflection (AR) layer (not shown). This is to reduce the so-called Fresnel reflection of light.
FIG. 2 is a schematic plan view of the above-described prior-art LCD device seen from the viewer's side. In FIG. 2(a), the light source 21 and the guide plate 22 for the front light are located on this side, and the reflector plate 113 is located behind them. Here, the angle of the prism array (i.e., the prism grooves) of the plate 22 with respect to the horizontal wiring lines of the panel 11 is defined as θ (θ≠0). In addition, FIG. 1 is a cross-sectional view along the line I—I in FIG. 2.
The reflector plate 113 has on its surface a specific surface irregularity, i.e., a specific reflection pattern formed by bumps and holes on the surface. The reflection pattern of the pixel PX 11 is schematically shown in FIG. 2(b). With the color LCD panel for displaying images in color, the pixel PX11 consists of three rectangular elements EL 11, EL 12, and EL 13 for Red (R), Green (G), and blue (B) colors. All the elements EL 11, EL 12, and EL 13 have the same reflection patterns.
FIG. 2(c) shows a conceptual model of reflection pattern of the pixel PX11 formed by the elements EL 11, EL 12, and EL 13. Since these elements EL 11, EL 12, and EL 13 have the same reflection patterns, the elements EL 11, EL 12, and EL 13 are painted out with the same color. The character “L” denotes the arrangement pitch of the pixels PX11.
As explained above, with the reflective type LCD device, information or image is usually displayed on the screen using ambient light, and the front light is required only in situations where no ambient light is expected. Therefore, it is important for the front light to be designed in such a way as not to degrade the characteristics and display quality of the panel 11.
First, a phenomenon to which we pay attention in connection with the display quality of the reflective type LCD device is “moiré (interference)” generated by the pitch P of the prism grooves of the guide plate 22 and the wiring lines of the panel 11. This phenomenon is referred to as the “first moiré” hereinafter.
The “first moiré” is recognizable regardless of whether the front light is turned on or not. To prevent the “first moiré” from occurring, generally, the following two design methods are adopted.
The first design method is a method that the prism groove pitch P of the guide plate 22 is equalized to the pixel pitch L of the panel 11 (i.e., P=L) and at the same time, the angle θ of the prism array (i.e., the prism grooves) of the plate 22 is set at zero (i.e., θ=0). The second design method is a method that the pitch P of the plate 22 and the angle θ of the prism array (i.e., the prism grooves) thereof are set in such a way as to satisfy the condition that the first moiré is not recognizable. In the second design method, there is an anxiety that bright lines and dark lines are observed and at the same time, the optical utilization efficiency is less than the first design method. Therefore, the first design method is usually adopted. Additionally, in the first design method, the stripe-shaped elements EL1, EL12, and EL13 of the pixel PX11 need to be located perpendicular to the prism array of the plate 22.
Second, there is a moiré generated in the state where the front light is being turned on, which is caused by the mirror reflection occurring directly below the guide plate 22. This moiré is referred to as the “second moiré” hereinafter.
As seen from FIG. 1, the front light forms a kind of diffraction grating. The light reflected by the reflection parts 22b of the plate 22 enters the LCD panel 11. In addition, for example, the light reflected by the surface of the polarizer section 12 enters the plate 22 again and thereafter, it propagates to the viewer's side by way of its propagation parts 22a. This means that this light passes through the said grating twice. Thus, the light having the zero-order and that having the first order induce optical interference, generating the “second moiré”.
A measure against the “second moiré”, i.e., the third design method, is to suppress the mirror reflection on the surface of the polarizer section 12 as much as possible. For example, the use of a polarizer plate having diffusibility on its surface will reduce the said mirror reflection. In addition, diffusion beads may be formed on the surface of the section 12 by the so-called “anti-glare process”, in which the light is scattered at the boundary between the surface of the section 12 and the atmospheric air to give the diffusibility.
By the way, with the above-described prior-art LCD device, as shown in FIG. 2, the irregularity pattern (i.e., reflection pattern) of the reflector plate 113 of the LCD panel 11 is formed at random in each of the elements EL11, EL12, and EL13, or in the pixel PX11. However, the patterns of the adjoining elements EL11 and EL12, or EL12 and EL13 are the same. This is because, for example, when an irregularity pattern is randomly formed in the whole panel 11, there is a possibility that the obtainable reflection characteristics are not uniform in the entire surface of the plate 113, even if random numbers are used.
Moreover, when different surface irregularity patterns are formed in the respective pixels, there is a possibility that the obtainable reflection characteristics are different between the adjoining R, G, and B elements in the adjoining pixels, which makes it difficult to control the reflection characteristics. Since irregularity patterns are minute, the basic pattern is preferably limited in a narrow area to a certain extent taking the efficiency of mask design for the patterns into consideration.
Furthermore, there is a “third moiré” observed when the front light is being turned on, which is generated by the irregularity pattern of the reflector plate 113 of the panel 11. Specifically, as seen from the schematic plan view of the plate 113 in FIG. 4, the surface irregularity pattern of the plate 113 is randomly formed in each element (or each pixel). Therefore, there is a possibility that the beams of light reflected by the adjoining bump and hole, the adjoining bumps, or the adjoining holes of the pattern cause optical interference.
If the above-described first design method (P=L, θ=0°) is adopted, as shown in FIG. 2, the elements (or the pixels) are repeatedly arranged in the same direction as the arrangement direction of the prism grooves of the guide plate 22 at the same period as the grooves, thereby promoting the “third moiré”. As a result, a moiré parallel to the prism grooves is formed. Actually, in the test conducted by the inventor, he prepared a TFT substrate where the reflector plate 113 was formed and then, he placed a front light fabricated by the first design method on the substrate. In this case, after turning the front light on, he observed colored interference light in the same direction as the prism array.
Similar to the measure against the second moiré, the third moiré can be reduced by giving diffusibility by way of surface treatment of the polarizer section 12. If so, however, there is a problem that contrast and recognizability degrade, when the pattern of the reflector plate 113 has less diffusibility (i.e., the pattern has many flat parts and high directivity). As a result, this measure is unable to suppress or eliminate the third moiré.
In addition to the said measure giving diffusibility by way of surface treatment of the polarizer section 12, there is a measure to improve the recognizability by way of filling the gap between the guide plate 22 and the section 12 with a resin. In this measure, however, the refractive index difference is decreased by the resin and thus, the light will be difficult to diffuse. As a result, the effect obtained by said measure giving diffusibility by way of surface treatment of the section 12 is not expected. This means that this measure is not applicable to suppress or eliminate the third moiré.
With the above-described first design method (P=L, θ=0°), the prism array of the guide plate 22 needs to be arranged in parallel to the horizontal wiring lines of the LCD panel 11. However, the condition of θ=0° is difficult to be realized due to the dispersion and clearance of the parts dimensions, or the assembly accuracy. If θ≠0°, the optical interference position deviates from its desired position and as a result, the third moiré will be recognized more easily.
The above-described explanation relates to the reflective type LCD device. However, there is a possibility that a similar moiré occurs in the “semi-transmissive type” LCD device, in which a back light is provided instead of the front light and information is displayed on the screen in ill-illuminated places by forming a penetration area in each element. When a back light is used, it is popular that a prism sheet and/or a prism guide plate is/are provided to raise the brightness. Therefore, due to the relationship between the periodicity of the prism grooves and the periodicity of the penetration areas, there is a possibility that optical interference (moiré) occurs.